The present invention relates to a resonant tag and a method of manufacturing a resonant tag. Particularly, the present invention is concerned with a resonant tag to be adhered to an item on sale for the confirmation of a robbery such as a shoplifting, and a method of manufacturing such a resonant tag.
Conventional resonant tags used for the purpose of preventing robbery such as shoplifting in supermarkets, speciality stores or department stores contain a resonant frequency circuit which is manufactured in the manner described below. Aluminum foils having different thicknesses are bonded by various bonding methods to the two surfaces of a synthetic resin film, such as a polyethylene, having a predetermined thickness and serving as a dielectric. Subsequently, a RL circuit pattern and a capacitor first circuit pattern are printed on the surface of the aluminum foil having a larger thickness by, for example, gravure printing, and a capacitor second circuit pattern is printed on the surface of the aluminum foil having a smaller thickness by the same printing method. Thereafter, etching is conducted on the aluminum foils using an alkali (caustic soda) or acid (ferric chloride) chemical.
It is possible to manufacture RLC resonant frequency by electrically connecting the circuit patterns on both sides of the synthetic resin film.
In the resonant tag manufactured in the manner described above, the synthetic resin film, such as polyethylene, used as the dielectric, is formed by melting a resin and then by forming the molten resin by the extrusion.
Hence, the thickness of the synthetic resin film such as polyethylene, which is used as the dielectric in the resonant tag, is not uniform in both the direction of extrusion and the direction transverse to it, non-uniformity depending on the precision of the extruder and the skill of the manufacturing technique. That is, the thickness of the synthetic resin film varies within a certain percentage range of the designed thickness.
When resonant tags are manufactured, a large number of resonant tags are arranged in rows and columns on a material which is as wide as possible to achieve reduction in the production cost. This makes the thickness of the dielectrics of all the tags arranged in rows and columns varied.
Variations in the resonant frequency of the resonant tag is largely affected by variations in the thickness of the dielectric of a capacitor circuit.
In consequence, variations in the thickness of the dielectrics of the respective resonant tags cause variations in the resonant frequencies of the respective resonant tags.
This may generate large amount of partially defective resonant tags whose resonant frequency is varied greatly due to the variations in the thickness of the dielectric in both directions of the arrangement, although the amount finally depends on the performance of an electronic detector for detecting passage of a resonant tag. That is, resonant tags whose resonant frequency cannot be detected by the electronic detector are generated. This greatly reduces the productivity and raises the production cost.
In the case of the resonant tags in which the electrode plates of the capacitor circuit patterns are located at the center of the RL circuit pattern (the coil), passage of the magnetic flux therethrough is interferred by the capacitor circuit patterns. Therefore, capacitor circuit patterns having small area electrode plates are desired to improve the performance of the resonant tag. The area of the electrode plates is not a factor which determines the general performance of the resonant tag but a factor which, together with the thickness of the dielectric, affects the capacitance of the capacitor circuit and thereby the resonance frequency.
In case the area of the electrode plates for the capacitor are reduced to enhance the performance, the thickness of the dielectric which corresponds to the capacitor circuit must be reduced as well to obtain the same resonant frequency.
However, manufacture contains various types of machinings including etching, so the resonant tag must have a sufficient mechanical strength which resists these machinings. Hence, there is a limitation to the reduction in the thickness of the dielectric such as the synthetic resin film.
Furthermore, the accuracy of the extruder is in inverse proportion to the thickness of the dielectric.
At present, the lower limit of the thickness of the synthetic resin film which can be used for manufacturing tags under the above conditions is 25 microns .+-.4 microns in terms of the precision of the extruder and the strength of the material required for the machinings.